In the art of keyless inking for lithographic printing processes whereby ink is metered into a printing press system by means of a metering roller and a cooperating scraping blade, Fadner in U.S. Pat. No. 4,601,202, Fadner and Hycner in U.S. Pat. No. 4,537,127 and Fadner in U.S. Pat. No. 4,603,634 have disclosed an advantageous method and means wherein the whole surface of a hard, wear-resistant metering roller for lithographic ink also possesses the dual property of being both hydrophobic and oleophilic, that is, water repelling and oil attracting. As clearly disclosed in Fadner, et. al., U.S. Pat. No. 4,690,055, one of the essential press components for operable keyless lithography is a metering roller having the aforementioned properties.
Hard ceramic materials such as chromium oxide, aluminum oxides and tungsten carbide are naturally high energy materials and correspondingly tend to be hydrophilic in the presence of water and tend to be oleophilic in the presence of oily materials. Metering rollers manufactured using these materials, while often used successfully in conjunction with either water based inks or with oil based inks in flexographic or letterpress printing respectively, fail to deliver consistent quantities of ink during lithographic printing wherein oil-based inks are used with water present. The extent of ink delivery inconsistency is determined by whether water present in the ink has displaced or debonded ink from the roller's ceramic surface As previously noted in U.S. Pat. No. 4,690,055, the extent of debonding depends in part upon the water content of the ink at any selected cross-press location, which water content in turn depends upon the format being printed.
The previously referred to U.S. Pat. No. 4,601,242 discloses one means for using the advantageously hard and wear-resistant ceramic property to obtain reasonably long lithographic ink metering roller lifetimes. Specifically, ceramic powder, and in particular alumina, is flame sprayed in a purposefully thin layer of less than 3 mils thickness over a copper-plated, celled, metering roller base. Copper is naturally hydrophobic and oleophilic. This procedure results in a hard, wear-resistant surface that has sufficient interparticle porosity relative to ink and water interactions so that the surface acts as if it was copper, thereby retaining ink in preference to water, yet simultaneously acting as a wear-resistant ceramic material relative to scraping blade wearing action. Although commercially viable, this type of roller may have a lifetime on a printing press of only about 20 to 30 million printing impressions because the ceramic layer must be kept relatively thin to assure that the hydrophobic property of the underlying copper is not negated by the hydrophilic characteristic of the ceramic layer. Further, the ceramic layer, which is naturally hydrophilic as well as oleophilic, may become increasingly or permanently hydrophilic due to an -accumulation of contaminants associated with use and cleaning of printing presses.
As disclosed in U.S. Pat. No. 4,977,830, hard ceramic coated rollers are known that can be treated to be beneficially contaminated, prior to or subsequent to the plasma spraying or laser engraving steps that produce the metering holes or cells in a ceramic coating, with oleophilizing essentially-organic compounds that more or less permanently render oleophilic and hydrophobic the somewhat porous, particulate ceramic surface of the resulting meter roller. This metering roller technology is useful in lithographic printing as long as the roller in fact remains oleophilic and hydrophobic as it is gradually worn due to the surface scraping action of the ink-doctoring blade. Two inherent drawbacks may limit this technology's useful lifetime on a press to, for instance, fifty million printed impressions or less.
One of these drawbacks is that laser engraving of high-melting-point ceramic materials leaves a hard recast layer of the ceramic material on the land areas surrounding each drilled hole. If this recast material is not completely removed, the ink delivery volume of the roller is somewhat unpredictable because the effective cell volume is greater than that predicted based on the nominal dimensions of the laser drilled holes or cells. The doctor blade rides on the recast mounds rather than on original surface land areas between the holes. Also, during printing these relatively rough or peaked recast material mounds act as force concentration centers to more rapidly wear out the co-acting doctor blade. Technology exists to remove all of the recast material by careful grinding. Accurately doing so involves the cost of an additional roller manufacturing step and requires removing at least some of the original ceramic coating to be certain of having removed all the recast material. This regrind approach can cause two problems relative to manufacture of hard ceramic rollers that for lithography need to be oleophilic and hydrophobic. One problem is that removal of the uppermost portions of the original surface-treated and therefore hydrophobic and oleophilic regions might destroy that essential dual surface property by inadvertent removal of the necessarily uppermost surface-treated roller portions. Secondly, surface grinding of hard materials such as ceramics involves the use of cutting fluid coolant/lubricants which may permanently adversely contaminate the metering roller upper surfaces destroying its hydrophobic and oleophilic properties.
The second inherent drawback of the laser-engraved approach to formation of metering cells when using ceramic or similarly hard, high-melting point materials is that the laser-drilled holes or cells are inherently conical in shape. Consequently, as the outermost surface portions of the roller are worn away during printing, even assuming a nominally smooth land area surface free of recast material was available initially, the cell volume decreases rather rapidly. Therefore, the amount of ink that can be delivered declines as the roller is worn. This negates consistent ink delivery volume over long time utilization of the roller on the press.
Mechanically engraved rollers are made by embossing well-defined patterns into a steel or aluminum pipe or cylinder roller core or base, then appropriately treating or overcoating with one or more thin layers of material to produce a chemically and mechanically resistant celled metering roller, as described in U.S. Pat. No. 4,862,799, for example. The nature of the continuous embossing process of a rotating curved cylinder surface requires that the walls of the cells be at a wide angle relative to the radial direction so that the embossing tool can enter and leave the cell as it is being created without fracturing the wall material. A typical cell geometry then is a truncated pyramid or similar configuration. All metering rollers manufactured by embossing engravature necessarily therefore have angular cell walls approximating the conical shape obtained with laser engraving of hard materials.
These inherent drawbacks of prior art metering roller technologies render difficult practical attempts to purposefully vary the amount of ink delivered by the celled metering roller, for instance by external application of modifying forces such as heat or pressure, to advantageously vary the printed optical density as required. If a metering roller could be manufactured that continued to deliver fixed amounts of ink over very long useful lifetimes on press despite its being gradually worn during that lifetime, it would be reasonable to consider using such independent means to vary ink delivery volume by controllably causing the ink input system to change from a known constant-delivery condition.
With flexographic or gravure keyless inking systems which use highly fluid inks, pigment content in the ink can readily be varied at press-side to accomplish the effect of delivering more or less coloration (pigment) to the substrate being printed. When using viscous oil-based lithographic inks, press-side alteration of the ink is generally not an acceptable alternative for practical operational reasons.
Changing to a metering roller having larger or smaller ink delivery capacity is another alternative for changing the ink input quantity and therefore the pigment delivery quantity, which therefore changes the printed optical density. This requires designing the press with quick-roller change capability as a criterion, often at the sacrifice of other machine or operational design options. Also, the metering rollers for large, high-speed presses are heavy, requiring mechanical lifting assistance devices. Such changes are generally not sufficiently rapid for use in high-speed, high volume printing operations. Means are needed to avoid these impractical means for modulation of keyless inking printed optical density values.